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Abstract
Polarised infrared reflection spectra of TlClO 4, TlBF 4 and NH 4BF 4 single-crystal plates were recorded and analysed for the frequencies of the transversal and longitudinal optical zone-centre phonon modes and other oscillator parameters by using the three-parameter (dielectric sum function) and the four-parameter (factorised form) oscillator-fit methods. In addition, single-crystal Raman spectra of the title compounds and the spectra of polycrystalline samples including those of 11B-enriched NH 4BF 4 were monitored. The obtained spectra are assigned and discussed with respect to (i) splitting of the respective modes due to all 10B/ 11B isotope substitution, long-range Coulomb forces (TO/LO), distortion of the anionic units (site group), and dynamic interaction (Davydov), (ii) phase transition to the cubic high-temperature polymorphs, and (iii) change of bonding (including effective charges) on going from both the thallium to the alkali-metal compounds and from perchlorates to tetrafluoroborates.
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